Catalyst system for ammoxidation of paraffins

ABSTRACT

Ammoxidation of C 3  to C 5  acyclic alkanes with NH 3  and O 2  using (1) a mole ratio of alkane:NH 3  in the range from 2 to 16 and a mole ratio of alkane:O 2  in the range 1 to 10 and (2) a mixture of particulate catalyst compositions, the first being especially effective to promote formation of an unsaturated nitrile and an olefin from the paraffin, and the second catalyst composition being especially effective to promote the conversion of the olefin to the unsaturated nitrile. Catalytic compositions useful in the process are disclosed.

This is a division of application Ser. No. 642,260, filed Aug. 20, 1984,now U.S. Pat. No. 4,637,976.

This invention relates to an improved process for the catalyticammoxidation of paraffins containing from 3 to 5 carbon atoms toα,β-unsaturated nitriles, especially paraffins containing 3 to 4 carbonatoms. Most important is the ammoxidation of isobutane tomethacrylonitrile and, especially, of propane to acrylonitrile.

Because of the price differential between propylene and propane aneconomic incentive exists for the development of a viable catalyticprocess for conversion of propane to acrylonitrile.

Earlier attempts in the prior art to develop an efficient process forthe ammoxidation of propane to acrylonitrile produced eitherinsufficient yields or processes that necessitated adding halogenpromoters to the feed. The latter procedure would require not onlyreactors made of special corrosion resistant materials, but also thequantitative recovery of the promoter. The added costs thus eliminatedthe advantage of the propane/propylene price differential.

It is thus an object of the present invention to provide an improvedprocess for the ammoxidation of paraffins to unsaturated nitriles.

It is a further object of the invention to provide new catalyst systemsfor such process.

Still another object is to provide an improved catalytic ammoxidationprocess for making unsaturated nitriles from lower paraffins without theuse of halogen promoters.

Other objects, as well as aspects, features and advantages, of thepresent invention will become apparent from a study of the accompanyingdisclosure and the claims.

The foregoing and other objects of the present invention are achieved bythe process of the present invention. There are two main features of thepresent process invention. The first of these is the use of an excess ofthe alkane feed with relation to NH₃ and molecular oxygen. The secondfeature, which is used in combination with the high ratio of the C₃ toC₅ paraffin to NH₃ and O₂, is that a combination, i.e., a mixture, ofcatalysts is employed, the first catalyst composition being especiallyeffective to promote formation of an unsaturated nitrile and an olefinfrom the paraffin, and the second catalyst composition being especiallyeffective to promote the conversion of the olefin to the unsaturatednitrile. Such mixture is the subject of the composition claims herein.

In the present application "paraffin" designates an acyclic paraffin.

British Patent Specifications Nos. 1,336,135 and 1,336,136 disclose theuse of high ratios of propane or isobutane to ammonia and oxygen, butonly single ammoxidation catalysts are used, and the yields ofacrylonitrile are extremely poor. U.S. Pat. No. 3,860,534 also disclosesuse of such high ratios, using a catalyst containing only V and Sboxides. However, after the catalyst is calcined, it is washed for 24hours with water and dried, a laborious procedure. A.N. Shatalova et al.in Neftekhiniya 8, No. 4, 609-612 (1968), describe the reaction ofpropane with oxygen and ammonia using a large excess of propane and amixture of two catalysts, one of which is described as oxides of metalshaving dehydrogenating characteristics at 550° and 600° C. At 500° C.little or no acrylonitrile was produced. Rather large amounts ofpropionitrile and acrolein were made per mole of acrylonitrile produced.The per pass conversion of propane to acrylonitrile was generally 2-4percent with selectivity to acrylonitrile being from 12 to 33 percent.

In the present process when applied to propane ammoxidation a smallamount of propylene is produced in relation to the unreacted propane inthe effluent. Such propane effluent containing propylene in the amountof up to 8 mole percent, but usually no more than 6 mole percent, of theamount of propane plus propylene can comprise the substrate feed to thepresent process. And in general the C₃ to C₅ alkane feed to the processcan contain one or more C₃ to C₅ olefins. The C₃ to C₅ olefin content ofthe feed to the present ammoxidation process can contain from zero to 8mole percent of such olefin(s), based on the moles of C₃ to C₅ paraffinplus olefins fed, and this feed can be from any source. Although largeramounts of C₃ to C₅ olefins may be present in the substrate paraffinfeed, usual amounts are as stated, and the usual olefin is thatcorresponding to the particular paraffin fed to the reaction zone of thepresent process.

According to the present invention there is provided a process for theammoxidation of a C₃ to C₅ paraffin which comprises contacting in areaction zone said paraffin in the vapor phase in admixture withammonia, molecular oxygen, and optionally an inert gaseous diluent, withan intimate particulate mixture of a first catalyst composition and asecond catalyst composition, said feed to the reaction zone containing amole ratio of paraffin:NH₃ in the range from 2 to 16 (usually 3-7), anda mole ratio of paraffin to O₂ in the range from 1 to 10 (usually1.5-5), said first catalyst composition being 10-99 weight percent of adiluent/support and 90-1 weight percent of a catalyst having thecomponents in the proportions indicated by the empirical formula:

    VSb.sub.m A.sub.a B.sub.b C.sub.c T.sub.t O.sub.x,         formula (1)

where

A is one or more of W, Sn, Mo, B, P and Ge;

B is one or more of Fe, Co, Ni, Cr, Pb, Mn, Zn, Se, Te, Ga, In and As;

C is one or more of an alkali metal and Tl;

T is one or more of Ca, Sr and Ba; and

where m is from 0.01 and up to 20; a is 0.2-10; b is 0-20; c is 0-20(usually 0-1); t is 0-20; the ratio (a+b+c+t):(l+m) is 0.01-6; wherein xis determined by the oxidation state of the other elements, and whereinthe antimony has an average valency higher than +3 and the vanadium hasan average valency lower than +5, said second catalyst composition being0-99 weight percent of a diluent/support and 100-1 weight percent of acatalyst having the components in the proportions indicated by theempirical formula:

    Fe.sub.10 Sb.sub.d Te.sub.e Me.sub.f G.sub.g O.sub.h R.sub.i O.sub.xformula ( 2)

where

Me is V, Mo, W or a combination of these;

G is one or more alkali metals;

Q is one or more of Cu, Mg, Zn, La, Ce, Cr, Mn, Co, Ni, Bi, Sn;

R is one or more of P, B and

d=10-60, e=0-10, f=0-6, g=0-5, h=0-10, i=0-5, and x is a numberdetermined by the valence requirements of the other elements present,and wherein the weight ratio in said mixture of said first catalystcomposition to said second catalyst composition is in the range of 0.001to 2.5.

In an especially useful catalyst of formula (1), m is greater than 1(often 2-10, more often 3-7).

By "particulate mixture" as used herein is meant a mixture of solidparticles or subdivided pieces of the first catalyst composition withseparate and distinct solid particles of the second catalystcomposition. The particles are often of a size used in fluidized bedreactors, say about 40 to 90 microns, but of course larger particles ofcatalyst can be employed for use in fixed or gravity flowing catalystbeds.

In the present process in all its embodiments the ratio of O₂ to NH₃ fedto the reaction zone is usually in the range from 1 to 10 (more often1-5) and the ratio of inert gaseous diluent to paraffin is usually inthe range zero to 5 (more often zero to 3).

The diluent or support for either catalyst composition is a refractorymetal oxide or mixture, such as silica, silica-alumina, etc.

In the usual practice of the present invention the catalystsupport/diluent for the catalyst of formula (1) is not an oxide of anelement named in formula (1). Further, in the usual practice of theinvention the catalyst support/diluent for the catalyst of formula (2)is not an oxide of an element named in formula (2).

In the catalyst compositions of the invention the catalyst empiricalformulas (1) and (2) do not, of course, connote any particular chemicalcompound, nor indicate whether the elements are present as a mixture ofindividual oxides or as a complex oxide or oxides, or what separatecrystalline phases or solid solutions may be present. Similarly, thedesignation of certain oxides, such as "silica" or "alumina" or Siw₂ orAl₂ O₃, as supports or diluents is merely in accordance with conventionin the inorganic oxide catalyst art, and such designations refer tocompounds often regarded as supports in the catalyst art. Suchdesignations, however, do not mean that the element involved is actuallypresent as a simple oxide. Indeed, such elements may at times be presentas a complex oxide with one, more than one, or all of the elements informula (1) or formula (2), which complex oxides form during theprecipitation or agglomeration, drying and calcining process forpreparing the catalyst composition.

The process of the invention is especially useful in the ammoxidation ofpropane or isobutane.

According to the present invention the foregoing first catalystcomposition is prepared under conditions such that in the finalcomposition the average oxidation state of vanadium is less than 5.

One method for preparing the first catalyst composition is by a redoxreaction between a compound of trivalent antimony such as Sb₂ O₃ and acompound of pentavalent vanadium such as V₂ O₅, during which theantimony is oxidized and the vanadium reduced.

The foregoing redox reaction was described by Birchall and Sleight(Inorganic Chem. 15, 868-70 [1976]) and by Berry et al. (J. Chem. Soc.Dalton Trans., 1983, 9-12), who effected the reaction by heating a drymixture of the above reactants at temperatures above 600° C. Thisproduct had a tetragonal rutile-type crystalline structure with a uniquex-ray diffraction pattern.

However, it has been found that the redox reaction can successfully andmore conveniently be carried out in an aqueous medium by heating at atemperature of at least 80° C. and up to 200° C., for instance, byheating an aqueous dispersion of a V⁵⁺ compound, such as NH₄ VO₃ or V₂O₅, with an Sb³⁺ compound, such as by reacting Sb₂ O₃ and NH₄ VO₃ (or V₂O₅). This step is followed by evaporation, drying and then calcining theproduct in a molecular oxygen-containing atmosphere, such as air, atfrom 350° to 700° or 750° C., usually 400° to 650° C. The length of thecalcination period may range from 30 minutes to 12 hours, butsatisfactory catalyst compositions are usually obtained by calcinationat such temperatures for a period of from 1 to 5 hours.

At least part of any excess of trivalent antimony compound, such as Sb₂O₃, is usually oxidized to Sb₂ O₄ during the calcination in a molecularoxygen-containing atmosphere, such as air.

The ingredients of the first catalyst composition other than vanadiumand antimony (and of course part of the oxygen() suitably can beincorporated after completion of the foregoing redox reaction. Thus, theadditives A, B, C and/or T, if any, can be added in the slurry after theredox reaction, or the solid particles containing the vanadium andantimony values after separation from the aqueous medium can be coatedor impregnated in a known manner with such additives at any suitablestage prior to final calcination of the catalyst, by methods generallyknown in the art, using oxides, hydroxides, acids, salts (particularlyorganic salts such as acetates), and other compounds of such elements.

In formula (1) subscript a usually is at least 0.4 or 0.5. In formula(1) at least 0.2 atoms of W are usually present per atom of V, and thetotal of W plus Sn atoms (if any Sn is present) is usually at least 0.4atoms. Preferred compositions of formula (1) contain at least 0.4 atomsof W per atom of V. Particularly when W is present, it is especiallyuseful to have at least 0.4 atoms of P per atom of V in addition to theW. Especially useful are such compositions wherein said diluent/supportcomprises 20-100 weight percent alumina and 80 to zero weight percentsilica.

Especially useful catalysts of formula (1) description are those inwhich a is at least 1, wherein A includes at least 1 atom of W.

Not only does the catalyst support in the first catalyst composition(formula (1) improve mechanical stability of the catalyst, but also thecatalytic activity is significantly improved, especially in the case ofalumina and silica-alumina. Besides alumina and silica-alumina othersupports that can be used are silica, titania, silica-titania, Nb₂ O₅,silica-niobia, silica-zirconia, zirconia and magnesia, etc.

In the first catalyst composition, now preferred support materials fornot only improving mechanical stability but also for improving the yieldof the desired nitriles are selected from silica-alumina and aluminahaving 20-100, usually 50-100, preferably 60-100 weight percent alumina;silica-titania and titania having 20-100 weight percent titania;silica-zirconia and zirconia having 80-100 weight percent zirconia; andsilica-niobia and niobia having 30-100 weight percent niobia (Nb₂ O₅).

In the preparation of the second catalyst composition of formula (2) themetal oxides can be blended together or can be formed separately andthen blended or formed separately or together in situ. Promoter oxides,if any, are preferably incorporated into the iron-antimony basedcatalyst by blending into the gel before calcining or by blending intothe oven-dried base catalyst before calcining. A preferred manner ofincorporating promoter elements is by choosing a water-soluble salt ofthe promoter element, forming an aqueous solution of the salt, andmixing the solution with a solution or a suspension of the base elementsor salts thereof. Optionally, the promoter elements can be incorporatedby the use of soluble complex salts or compounds with the desired baseelements which upon calcination will yield the desired ratio of theelements in the finished catalyst.

To introduce the iron component into the catalyst one may use anycompound of iron which, upon calcination, will result in the oxides. Aswith the other elements, water soluble salts are preferred for the casewith which they may be uniformly dispersed within the catalyst. Mostpreferred is ferric nitrate. Cobalt and nickel are similarly introduced.

Antimony is suitably introduced as Sb₂ O₃, or an Sb₂ O₅ sol can be used.It is also possible to add finely divided antimony metal to concentratednitric acid and then to boil the slurry to decompose the excess nitricacid, leaving an antimony oxide slurry.

Other variations in starting materials will suggest themselves to oneskilled in the art, particularly when the preferred starting materialsmentioned hereinabove are unsuited to the economics of large-scalemanufacture. In general, any compounds containing the desired catalystcomponents may be used provided that they result, upon heating to atemperature within the range disclosed hereinafter, in the oxides of theinstant catalyst.

These second catalyst compositions are conveniently prepared by slurrytechniques wherein an aqueous slurry containing all of the elements inthe objective catalyst is produced, the water removed from the aqueousslurry to form a precatalyst precipitate or powder and the precatalystthen heated in the presence of an oxygen-containing gas such as air atelevated temperature to calcine the precatalyst thereby forming thecatalyst. Liquids other than water, such as C₁ to C₈ alcohols can alsobe used to form the precatalyst slurry.

In the second catalyst composition the support can be any of the usualsupports such as silica, alumina, silica-alumina, titania, zirconia, andNb₂ O₅.

In the ammoxidation of the present invention, the reaction is carriedout in the gas phase by contacting a mixture of the paraffin, ammoniaand molecular oxygen, and inert diluent, if any, conveniently in a fixedbed of the catalyst mixture, or a gravity flowing bed, a fluidized bedor a fast transport reactor mode.

Examples of inert diluents useful in the reaction are N₂, He, CO₂, H₂ Oand Ar.

The reaction temperature range can vary from 350° to 700° C., but isusually 430° to 520° C. The latter temperature range is especiallyuseful in the case of propane ammoxidation to acrylonitrile.

The average contact time can often be from 0.01 to 10 seconds, but isusually from 0.02 to 10 seconds, more usually from 0.1 to 5 seconds.

The pressure of the reaction usually ranges from 2 to 45 psia. Mostoften, pressure is somewhat above atmospheric.

The following examples of the invention are exemplary and should not betaken as in any way limiting.

EXAMPLE 1

A catalyst having the empirical formula 50 wt % VSb₅ WO_(x) +50 wt % Al₂O₃ support was made as follows:

In a stirred flask equipped for heating under reflux, 5.4 g NH₄ VO₃ wasdissolved in 150 ml hot water. To the hot solution 33.6 g Sb₂ O₃ wasadded, and the slurry was boiled under reflux for 16-18 hours overnight.There was ammonia evolution, and the vanadium antimony oxide mixtureturned gray-green.

In a separate operation, 59.0 g Catapal SB (hydrated alumina) was mixedwith 200 ml H₂ O (cold)+23.0 g acetic acid (10 percent solution) andstirred until the suspension gelled. It took about 3 hours, and the gelwas soft, homogeneous, with the consistency of thick cream.

Meanwhile, the vanadium antimony slurry was transferred to a beaker. Asolution of 12.5 g ammonium meta-tungstate in about 25 ml H₂ O was thenadded, followed by the addition, with stirring (magnet) of the aluminagel. After partial evaporation, the mixture became too thick forstirring. It was then transferred to an evaporating dish, and theevaporation, following by drying overnight, was continued in an oven at110°-120° C. The dried material was precalcined at 350° C. for 5 hours,screened to 20/35 mesh, then calcined 3 hours at 610° C.

EXAMPLE 2

A catalyst having the empirical formula 50 wt % VSb₃.5 P₀.5 WO_(x) +50wt % Al₂ O₃ support was made as follows:

In a stirred flask equipped for heating under reflux, 3.81 g NH₄ VO₃were dissolved in 90 ml hot water. To the hot solution 16.6 g Sb₂ O₃were added, and the slurry was boiled under reflux for 16-18 hoursovernight. There was ammonia evolution, and the vanadium antimonymixture turned gray-green.

In a separate operation, 35.3 g Catapal SB (hydrated alumina) were mixedwith 127.2 ml H₂ O (cold)+14.1 g acetic acid (10 percent solution) andstirred until the suspension gelled. It took about 3 hours, and the gelwas soft, homogeneous, with the consistency of thick cream.

Meanwhile, the vanadium antimony slurry was transferred to a beaker. Asolution of 8.80 g ammonium meta-tungstate in about 20 ml H₂ O and asolution of 1.77 g (NH₄)₂ HPO₄ in H₂ O were then added, followed by theaddition, with stirring (magnet) of the alumina gel. After partialevaporation, the mixture became too thick for stirring. It was thentransferred to an evaporating dish, and the evaporation, following bydrying overnight, was continued in an oven at 110°-120° C. The driedmaterial was precalcined at 350° C. for 5 hours, screened to 20/35 mesh,then calcined 3 hours at 610° C.

EXAMPLE 3

88.36 g of Fe(NO₃)₃.9H₂ O were dissolved in 170 cc of water withstirring; this solution was added to 687.86 g of a 12 weight percent Sb₂O₅ sol with stirring. A cloudy, light brown gel formed which broke upwith rapid stirring. The dispersion was concentrated by heating and thendried in an evaporating dish overnight in a drying oven at 130° C.Thereafter, it was heat treated at 290° C. for 3 hours and at 425° C.for 3 hours, then calcined by heating at 610° C. for 3 hours. Thecatalyst was ground to 20-35 mesh size. Its composition was Fe₃ Sb₇O_(x).

EXAMPLE 4

A catalyst having the empirical composition 60 wt % Fe₁₀ Sb₂₀ Cu₃.2Te₁.36 Mo₀.57 W₀.083 V₀.15 O_(x) +40 wt % SiO₂

Solution 1:

38.4 lbs of dionized water (D.I. H₂ O) was added to a container andheated to 80° C. Then 88.9 g of ammonium paratungstate was dissolvedtherein and the temperature lowered to about 50°-60° C. Then 72.8 g ofammonium metavanadate was dissolved in this solution and the temperaturewas then lowered to 35°-40° C. Later, 406 g of ammonium paramolybdatewas dissolved in the solution and the temperature was adjusted to30°-35° C.

Solution 2:

To another container were added 120 lbs of a metal nitrate aqueoussolution containing in weight percent: 4.27% Fe, 1.49% Cu, 1.30% Te and23.7% --NO₃. To this solution were added 808 pounds of 70.2 weightpercent HNO₃.

To a 50 gallon vessel, having a reflux condenser and a stirrer wereadded 71.8 pounds of D.I. H₂ O and 71.8 pounds of a 40 weight percentsilica sol (NH₃ stabilized). While stirring all of Solution 2 was addedand stirred for 5 minutes. Then 26.3 pounds of Sb₂ O₃ were added, andthen all of Solution 1 was added and stirring was continued for 5minutes.

Thereafter, over a period of 50-60 minutes, 45.8 pounds of 14.7 weight %aqueous NH₃ solution were added while keeping the temperature at 32°-38°C. The pH was measured at about 40° C. and was found to be 2.35.

The slurry was then heated to reflux (about 100° C.) over a period of 1hour, and thereafter was refluxed for 4 hours. Thereafter, a portion ofthe slurry was transferred to an evaporating dish, and the evaporationand drying was effected in an oven overnight at 110°-120° C. The driedmaterial was heated at 290° C. for 3 hours and 425° C. for 3 hours, thenground to 20-35 mesh and calcined by heating at 825° C. for 3 hours.

In the ammoxidation runs of the following examples, the catalyst, or themixture of catalysts, is in a tubular 3/8 inch I.D. stainless steelfixed bed reactor. When a mixture of particulate catalysts is used, asin the invention examples, the desired weight of each of the twocatalyst compositions is put in a vial and shaken until uniformlydispersed before placing the desired amount of the catalyst mixture inthe reaction tube. The reaction is equipped with a preheat leg and isimmersed in a temperature controlled molten salt bath. The gaseous feedcomponents are metered through mass flow controllers into the bottom ofthe reactor through the preheat leg. Water is introduced through aseptum at the top of the preheat leg, using a syringe pump. The feed isfed to the catalyst for 1 hour before collection of product; the runs ofeach example last 30 minutes during which the product is collected foranalysis.

EXAMPLE 5

In this example the catalyst was a mixture of the catalyst of Example 1and the catalyst of Example 4 in the weight ratio of the former to thelatter of 0.20. The reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The contact time was 1.8seconds. Analysis of the reactor effluent showed that propane conversionwas 12.0 percent; yield and selectivity of propane to acrylonitrile were2.8 and 23.3 percent, respectively; selectivity to propylene was 38.3percent.

COMPARATIVE EXAMPLE A

In this example the reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The catalyst was the catalystof Example 1 alone. The contact time was 0.2 seconds. Analysis of thereactor effluent showed that propane conversion was 16.9 percent; yieldand selectivity of propane to acrylonitrile were 3.3 and 19.5 percentrespectively; selectivity to propylene was 50.6 percent.

EXAMPLE 6

In this example the catalyst was a mixture of the catalyst of Example 2and the catalyst of Example 3 in the weight ratio of the former to thelatter of 0.15. The reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The contact time was 2.2seconds. Analysis of the reactor effluent showed that propane conversionwas 14.5 percent; yield and selectivity of propane to acrylonitrile were5.3 and 36.5 percent, respectively; selectivity to propylene was 27.4percent.

COMPARATIVE EXAMPLE B

In this example the reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The catalyst was the catalystof Example 2 alone. The contact time was 0.3 seconds. Analysis of thereactor effluent showed that propane conversion was 18.3 percent; yieldand selectivity of propane to acrylonitrile were 3.7 and 20.4 percent,respectively; selectivity to propylene was 52.8 percent.

COMPARATIVE EXAMPLE C

In this example the reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The catalyst was the catalystof Example 3 alone. The contact time was 0.8 seconds. Analysis of thereactor effluent showed that propane conversion was 2.9 percent; yieldand selectivity of propane to acrylonitrile were 1.2 and 42.1 percent;selectivity to propylene was 15.9 percent.

EXAMPLE 7

In this example the catalyst was a mixture of the catalyst of Example 2and the catalyst of Example 4 in the weight ratio of the former to thelatter of 0.15. The reaction temperature was 470° C. and the molar feedratios were 5 propane/1 NH₃ /2 O₂ /1 H₂ O. The contact time was 1.5seconds. Analysis of the reactor effluent showed that propane conversionwas 10.6 percent; yield and selectively of propane to acrylonitrile were3.5 and 32.4 percent, respectively; selectivity to propylene was 34.0percent.

As will be evident to those skilled in the art various modifications ofthis invention can be made or followed in the light of the foregoingdisclosure and discussion without departing from the spirit and scope ofthe disclosure or from the scope of the claims.

We claim:
 1. A catalytic mixture suitable for the ammoxidation ofpropane to acrylonitrile, which comprises an intimate particulatemixture of a first catalyst composition and a second catalystcomposition, said first catalyst composition being 10-99 weight percentof a diluent/support and 90-1 weight percent of a catalyst having thecomponents in the proportions indicated by the empirical formula:

    VSb.sub.m A.sub.a B.sub.b C.sub.c T.sub.t O.sub.x,         formula (1)

where A is one or more of W, Sn, Mo, B, P and Ge; B is one or more ofFe, Co, Ni, Cr, Pb, Mn, Zn, Se, Te, Ga, In and As; C is one or more ofan alkali metal and Tl; T is one or more of Ca, Sr and Ba; andwhere m isfrom 0.01 and up to 20; a is 0.2-10; b is 0-20; c is 0-1; t is 0-20; theratio (a+b+c+t): (1+m) is 0.01-6; wherein x is determined by theoxidation state of other elements; A includes at least 0.2 atoms of Wper atom of V, the antimony has an average valency higher than +3 andthe vanadium has an average valency lower than +5, and wherein saidsupport for the catalyst of formula (1) is selected from silica-aluminaand alumina having 20-100 weight percent alumina; silica-titania andtitania having 20-100 weight percent titania; silica-zirconia andzirconia having 80-100 weight percent zirconia; and silica-niobia andniobia having 30-100 weight percent niobia (Nb₂ O₅), said secondcatalyst composition being 0-99 weight percent of a diluent/support and100-1 weight percent of a catalyst having the components in theproportions indicated by the empirical formula:

    Fe.sub.10 Sb.sub.d Te.sub.e Me.sub.f GgQ.sub.h R.sub.i O.sub.xformula ( 2)

where Me is V, Mo, W or a combination of these; G is one or more alkalimetals; Q is one or more of Cu, Mg, Zn, La, Ce, Cr, Mn, Co, Ni, Bi, Sn;R is one or more of P, B and d=10-60, e=0-10, f=0-6, g=0-5, h=0-10,i=0-5, and x is a number determined by the valence requirements of theother elements present, and wherein the weight ratio in said mixture ofsaid first catalyst composition to said second catalyst composition isin the range of 0.001 to 2.5.
 2. A mixture of claim 1 wherein A includesat least 0.4 atoms of W per atom of V.
 3. A mixture of claim 2 wherein Aincludes a least 0.4 atoms of P per atom of V.
 4. A mixture of any oneof claims 1, 2 and 3 wherein m is 2-10.
 5. A mixture of any one ofclaims 1, 2 and 3 wherein said diluent/support in said first catalystcomposition comprises 20-100 weight percent alumina and 80 to zeroweight percent silica.
 6. A mixture of claim 5 wherein saiddiluent/support in said first catalyst composition comprises 50-100weight percent alumina and 50 to zero weight percent silica.